不能约束啊

<?xml version="1.0" encoding="UTF-8"?>

<!-- 教学计划课程信息 -->

<!DOCTYPE xml_2 SYSTEM "xml_2.dtd">

<teaching-plan>

  <course no="1">

    <course-name>大学英语</course-name>

    <class-hour>36</class-hour>

    <exam-form>考试</exam-form>

  </course>

  <course no="2">

    <course-name>高等数学</course-name>

    <class-hour>70</class-hour>

    <exam-form>考试</exam-form>

  </course>

  <course no="3">

    <course-name>计算机应用基础</course-name>

    <class-hour>108</class-hour>

    <exam-form>上机考试</exam-form>

    <a>www<a>

  </course>

</teaching-plan>

<?xml version="1.0" encoding="UTF-8"?>

<!ELEMENT teaching-plan (course+)>

<!ELEMENT course (course-name,class-hour,exam-form)>

<!ATTLIST course no CDATA "">

<!ELEMENT course-name (#PCDATA)>

<!ELEMENT class-hour (#PCDATA)>

<!ELEMENT exam-form (#PCDATA)>